1. Field of the Invention
The invention relates to a method and apparatus for making a spark plug for igniting a fuel gas in an internal combustion engine and more specifically relates to a method and apparatus for precisely making a spark gap between a center electrode and a ground electrode in the spark plug.
2. Description of the Related Art
In recent years, an allowable deviation of the spark gap between a firing end of the spark plug and a metal strip of the ground electrode has been narrowing due to recent requirements for a high performance spark plug usable e.g. in a maintenance-free internal combustion engine. Therefore, contrivances are required to provide a reliable way for making such an accurate distance for the spark gap by precisely bending the ground electrode strip.
In a conventional gap-forming process, the ground electrode strip is bent in a single step by applying a punch to the metal strip that is welded upright to an end face of a metal shell of the spark plug with a spacer inserted between the center electrode and the ground electrode. However, a statistical uniformity of the narrowly-deviated gap distance is not easily attained from one spark plug gap to the other, probably due to various factors such as buckling of the metal strip per se, a welding condition for the strip, deviations of the strip diameter and length, and an extent of projection of the firing end of the center electrode from the metal shell.
Specifically, a high performance spark plug which uses a high-corrosion resistant hard metal tip made from precious metal such as Platinum and/or Iridium having a small diameter of 0.3-1 mm formed on the center electrode will only tolerate a very small deviation of the spark gap from the optimum spark gap. Due to the metal tip additionally welded to the center electrode, e.g. by laser-welding, the extent of projection of the firing end may differ from one spark plug to another. Such a high performance spark plug is described, for instance, in EP 0 872 928, U.S. Pat. No. 5,793,793 and U.S. Pat. No. 5,977,695.
If the spacer is made to abut against such a small- diameter metal tip of the center electrode so as to minimize the deviation, a high compressive force is applied to the metal strip through the spacer as a result of bending the metal strip, which may lead to a possible risk that the small diameter tip welded to the center electrode may crack, be damaged or come off.
It is therefore an object of the invention to provide a method and apparatus for making a spark plug with a precisely formed spark gap between a ground electrode and a center electrode. It is another preferred object of the invention to provide a method and apparatus for making a precisely formed spark gap between a ground electrode and a firing end of a precious metal tip of a center electrode in a spark plug.
According to the invention, there is provided a method for manufacturing a spark plug comprising a center electrode disposed in a bore formed in a ceramic insulator, a metallic shell fitting outside the ceramic insulator, and a ground electrode forming a spark gap with the center electrode, the method comprising steps of:
extending a metal strip from an end of the metal shell;
positioning a spacer above a firing end of the center electrode;
preliminarily bending the metal strip toward the spacer so as to form an arc portion of the metal strip;
and then precisely forming a gap-distance between the metal strip and the firing end of the center electrode by further applying a force to the metal strip in a parallel direction with a center electrode axis.
In this specification, the phrase xe2x80x9cmetal stripxe2x80x9d refers to any elongate metal member, not limited to any specific cross-sectional shape or aspect ratio.
In one aspect of the invention, it is important that there is provided a spacer above the firing end so that the firing end is protected from being damaged during the step of preliminarily bending the metal strip. In another aspect of the invention, the position of the spacer is determined by referring to the position of the firing end of the center electrode so that the firing end of the center electrode does not touch the firing end of the center electrode and can form the arc portion in the metal strip. In other words, it is preferable that a clearance is provided between the spacer and the firing end of the center electrode so as to protect the firing end.
The arc portion formed in the metal strip assures leveling of a lateral side of the metal strip with the firing end face of the center electrode in the step of precisely forming the gap-distance between the metal strip and the firing end of the center electrode. The spacer has a rounded portion so that the arc portion in the metal strip is made between the ends of the strip by bending the metal strip toward or rather along the rounded portion by e.g. a punch. Instead of the rounded portion of the spacer, a corner-chamfered portion may accomplish the same purpose. It is best to make the arc portion at about the same level as the firing end face of the center electrode.
In a preferred aspect of the invention, the above method may further comprise a step of:
measuring the position of the firing end of the center electrode so as to determine a position for the spacer.
The measurement of the position of the firing end of the center electrode is importantly conducted before positioning the spacer so that the clearance between the spacer and the firing end of the center electrode is formed and maintained before and during bending the metal strip toward the spacer which is protecting the firing end of the center electrode.
In the same way as above, the firing end of the center electrode that has a firing tip having a small diameter, such as in the range of from 0.3 to 1.0 mm, is protected. The tip is preferably formed from metal selected from a group consisting of Pt, Ir, Rh, Pd, Re, Os, Ru or alloy thereof and is welded on the center electrode to form the firing end of the center electrode.
In a further preferred aspect of the invention, either of the above defined methods may further comprise a step of:
retrieving the spacer after bending the metal strip so that the gap-distance between the metal strip and the center electrode is adjusted to a required value by referring to a position of the preliminary bent metal strip and the position of the end of the firing metal.
In this preferred embodiment above, it is advantageous to retrieve the spacer after preliminarily bending the metal strip, because the gap distance between the metal strip (namely the ground electrode) and the firing end of the center electrode is easily measured or determined, for example, by a computer-controlled visual image processor that outputs to a metal-strip bending machine how much the gap distance between the ground electrode and the center electrode should be further narrowed or adjusted based on a side-view measurement of the gap distance.
In a preferred aspect of the invention, it is useful that information relating to the position of the firing end of the center electrode is stored in a computer memory so that the information is used for positioning the spacer. This information can be advantageously used also for the step that follows, in which step a gap-distance between the metal strip and the firing end of the center electrode is precisely formed or adjusted.
In yet another aspect of the invention, positional information is determined with reference to a position of a part constituting the spark plug. If a reference position is taken from an end of the metal shell by e.g. a position-detecting sensor using laser, the position of the firing end means how much the firing end of the center electrode is projected from the end of metal shell to which the metal strip is extendedly welded, thereby giving the positional information that directs how much above the spacer should be placed from the firing end of the center electrode and/or whereat the arc portion is formed in the metal strip.
In still another aspect of the invention, the positional information of the firing end of the center electrode can be used not only for positioning the spacer above the firing end as described above but also for precisely forming a gap-distance between the metal strip and the firing end of the center electrode. In this case, positional information is only required for the bent metal strip (of the ground electrode).
In a further aspect of the invention, the force applied to the metal strip for precisely forming the gap distance is caused by a punch or die moving in parallel with a center electrode axis of the spark plug. Alternatively, the preliminary bending of the metal strip may be done by a punch or die moving in perpendicular to the center electrode axis, or in a combined direction having both of these directions.
The present invention further provides an apparatus adapted to perform any of the above methods.